In analyzers which have a dispensing nozzle for aspirating and discharging liquid such a blood sample, reagent and the like, it is important to prevent contamination by a previously aspirated liquid in order to ensure the accuracy of the analysis when another new liquid is aspirated after the previously aspirated liquid was discharged. Therefore, for example, Japanese Laid-Open Patent Publication No. H6-3364 discloses a dispensing device in which a disposable pipette tip is installed on the tip of a dispensing nozzle, and after use the disposable pipette tip is removed and replaced by a new pipette tip.
The dispensing device disclosed in Japanese Laid-Open Patent Publication No. H6-3364 separates the pipette tip from the dispensing nozzle through impacting the top surface of the pipette tip on a releasing plate by inserting the dispensing nozzle, which has installed the pipette tip, into the notched part of the releasing plate and thereafter moving up the dispensing nozzle.
However, in the art disclosed in Japanese Laid-Open Patent Publication No. H6-3364, liquid that remains in the pipette tip is sometimes scattered when the pipette tip is separated by impacting the top surface of the pipette tip on the releasing plate, and the scattered liquid may adhere to the tip of the dispensing nozzle. In this case, the liquid may also adhere to the aspirating hole provided in the tip of the dispensing nozzle. When residual liquid in the pipette tip adheres to the tip of the dispensing nozzle, the liquid used in a subsequent analysis may be contaminated even though the pipette tip is replaced, thereby the accuracy of the analysis may be impaired.
In order to solve this problem, the dispensing device disclosed in Japanese Laid-Open Patent Publication No. 11-248714 looses the connection between the pipette tip and the nozzle part by lowering the nozzle part so as to bring the tapered part of the pipette tip into contact with an pressing plate which is located at a position slightly distanced from the center axis of the nozzle part and applying a pressing force in a horizontal direction on the pipette tip by the pressing plate, when separating the pipette tip from the nozzle base. Thereafter, the pipette tip is smoothly separated from the nozzle part by moving up the nozzle part and impacting the top surface of the pipette tip on an ejection plate.
In recent years, there has been a trend to reduce the quantity of samples such as blood collected from a patient to minimize the burden on the patient, and there has likewise been a demand for analyzers to reduce the amount of sample required for analysis. A further desire for high performance analyzers has arisen in conjunction with the increasing number of samples to be measured by the analyzers.
It is necessary to improve capability of the dispensing apparatus to dispense minute amount of sample in order to perform high accuracy analysis with a reduced amount of sample. Therefore a long and narrow pipette tip has been used recently. When such a long and narrow pipette tip is used and the pipette tip is separated from the dispensing nozzle, the angle formed by the tapered part of the pipette tip at the leading end and the perpendicular axis becomes extremely narrow. When the art disclosed in Japanese Laid-Open patent Publication No. 11-248714 is applied to the dispensing device using this type of pipette tip, problems arises such as (1) the position of the leading end of the descending pipette tip and the position of the pressing plate must be set in proximity in order to apply a horizontal force to the pipette tip by the pressing plate in conjunction with the descent of the pipette tip, because due to the narrow angle of the tapered part of the pipette tip. Therefore, high precision components and precision assembly are demanded for the pressing plate, and high precision control is demanded when lowering the nozzle on which the pipette tip is installed, (2) the horizontal force applied to the descending pipette tip by the pressing plate becomes weak due to the narrow angle of the tapered part of the pipette tip, and the residual liquid is scattered when the top surface of the pipette tip impacts on the ejection plate, and (3) causing problems (1) and (2) in conjunction with speeding up of the dispensing device.